U.S. patent number 5,726,534 [Application Number 08/636,263] was granted by the patent office on 1998-03-10 for preheat current control circuit based upon the number of lamps detected.
This patent grant is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Maeng-ho Seo.
United States Patent |
5,726,534 |
Seo |
March 10, 1998 |
Preheat current control circuit based upon the number of lamps
detected
Abstract
A feedback control system of ballast which has function to
detect the number of lamp and is applied to an integrated circuit
to control the ballast for a fluorescent lamp etc, and provides the
ballast with the feedback control system which can detect the
number of lamp, control ballast continuously by means of the n-lamp
detector and soft start controller which produce the compensated
current from the feedback current and direct link voltage.
Therefore, the feedback control system can control the ballast
accurately according to the load change such as the change of input
voltage, number of lamp.
Inventors: |
Seo; Maeng-ho (Kyonggi-do,
KR) |
Assignee: |
Samsung Electronics Co., Ltd.
(Suwon, KR)
|
Family
ID: |
19416539 |
Appl.
No.: |
08/636,263 |
Filed: |
April 24, 1996 |
Foreign Application Priority Data
|
|
|
|
|
Jun 5, 1995 [KR] |
|
|
95-14844 |
|
Current U.S.
Class: |
315/97; 315/105;
315/107; 315/308 |
Current CPC
Class: |
H05B
41/39 (20130101); H05B 41/392 (20130101) |
Current International
Class: |
H05B
41/392 (20060101); H05B 41/39 (20060101); H05B
037/02 () |
Field of
Search: |
;315/97,106,107,308,307,DIG.7,105 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pascal; Robert
Assistant Examiner: Shingleton; Michael
Attorney, Agent or Firm: Cushman Darby & Cushman IP
Group of Pillsbury Madison & Sutro LLP
Claims
What is claimed is:
1. A switching type ballast feedback control system,
comprising:
a ballast which generates a feedback current;
a detector which detects a number of lamps connected to the
ballast;
a reference voltage generator which generates a reference voltage
corresponding to the number of lamps detected by the detector;
a soft start controller unit which produces a first output current
corresponding to the number of lamps detected by the detector in an
initial preheat period, an instantaneous discharge period and a
continuous discharge period;
a first feedback unit which multiplies said feedback current of the
ballast with a direct link current applied to the ballast to obtain
a multiplied current that is added to the first output current of
the soft start controller unit to form a second output current;
a second feedback unit which converts the second output current
into a voltage, produces an error voltage as a difference between
the converted voltage and the reference voltage generated by the
reference voltage generator and converts the error voltage into a
third output current;
a main control unit which adds the third output current of the
second feedback unit to a feedforward current and determines a
control frequency of a driving signal of the ballast from this
added current.
2. The control system of claim 1, wherein the soft start controller
unit comprises:
a time controller which produces a voltage in proportion to the
time;
a soft start controller which produces current according to the
number of lamps in the initial preheat period, instantaneous
discharge period, and continuous discharge period, which periods
are distinguished by the output voltage of the time controller.
3. The control system of claim 1, wherein the main control unit
comprises:
an inductor circuit to feedforward the direct link voltage supplied
to the ballast;
a control block which adds the third output current of the second
feedback unit to the feedforward current of the inductor circuit
and determines the control frequency of the driving signal from
this added current.
4. The control system of claim 1, wherein the first feedback unit
comprises:
a multiplier which produces a multiplied current by multiplying the
feedback current from the ballast with the direct link voltage;
and
an adder which adds the output current of the multiplier to the
second output current of the soft start controller unit.
5. The control system of claim 1, wherein a second feedback unit
comprises:
a current to voltage convertor which converts the second output
current of the first feedback unit into voltage;
an adder which produces the error voltage by deducting the voltage
of the current to voltage convertor from the reference voltage from
the reference voltage generator;
a voltage to current convertor which converts the error voltage of
the adder into current.
6. The control system of claim 1, wherein
the detector comprises n comparators which detect the existence of
lamps by comparing sensed voltages with internal voltages and
output n output voltages;
an addition unit which adds the n output voltages of the n
comparators and outputs the added voltage to the reference voltage
generator and the soft start controller.
7. The control system of claim 2, wherein the soft stare controller
comprises:
a current source to supply the divided current to n cells in
circuit;
n cells which operate according to the output voltage of the n-lamp
detector, said n cells producing a current corresponding to the
output voltage of the n-lamp detector in proportion to the input
time of the time controller, such that said n cells produce a
constant current during the initial preheat period, produce a
proportionally decreasing current during the instantaneous
discharge period, and produce zero current during the continuous
discharge period.
8. The control system of claim 7, wherein the current source
comprises a current supplying unit having a transistor and wherein
each of said n cells has a transistor in mirror relation with said
transistor of said current supplying unit to divide said current to
said n cells.
9. The control system of claim 7, wherein each of said n cells
comprises:
a first transistor having a base, an emitter and a collector, the
base of said first transistor being connected to the divided
current from the current supplying unit;
a second transistor having a base, an emitter and a collector, the
base and collector of said second transistor being joined and the
collector of said second transistor being connected to the
collector of said first transistor;
a third transistor having a base, an emitter and a collector, the
base of said third transistor being is connected to the base of
said second transistor and the collector of said third transistor
being connected to the divided current from the current supplying
unit;
a fourth transistor having a base, an emitter and a collector, the
base of said fourth transistor being connected to the reference
voltage to determine the discharge period, the collector of said
fourth transistor being connected to the divided current from the
current supplying unit, and the emitter of said fourth transistor
being connected to the collector of said third transistor;
a fifth transistor having a base, an emitter and a collector, the
emitter of said fifth transistor being connected to the collector
of said fifth transistor through the resistor, the collector of
said fifth transistor is connected to the emitter of said first
transistor, and the base of said fifth transistor being connected
to a voltage in proportion to the time controller;
a sixth transistor having a base, an emitter and a collector, the
base of said sixth transistor being connected to the output voltage
of the n-lamp detector, the collector and emitter of said sixth
transistor being respectively connected to the base and emitter of
said third transistor and controlling the turn-off of said third
transistor according to the output voltage of the n-lamp
detector.
10. The control system of claim 3, wherein the control block
comprises:
an integrator which integrates the third output current of the
second feedback unit and produces an output voltage;
a voltage-controlled current source which produces an output
current according to the output voltage of the integrator;
an adder which adds the feedforward current of the inductor circuit
to the reference current through the control block to form an added
current and produces a total current by deducting the output
current of the voltage-controlled current source from the added
current;
an oscillator and driving circuit which receives the total current
from the adder and compares the total current with an internal
reference current and produces a driving signal to drive the
switching element of the ballast by dividing the total current and
the internal reference current.
11. The control system of claim 8, wherein each of said n cells
comprises:
a first transistor having a base, an emitter and a collector, the
base of said first transistor being connected to the divided
current from the current supplying unit;
a second transistor having a base, an emitter and a collector, the
base and collector of said second transistor being joined and the
collector of said second transistor being connected to the
collector of said first transistor;
a third transistor having a base, an emitter and a collector, the
base of said third transistor being is connected to the base of
said second transistor and the collector of said third transistor
being connected to the divided current from the current supplying
unit;
a fourth transistor having a base, an emitter and a collector, the
base of said fourth transistor being connected to the reference
voltage to determine the discharge period, the collector of said
fourth transistor being connected to the divided current from the
current supplying unit, and the emitter of said fourth transistor
being connected to the collector of said third transistor;
a fifth transistor having a base, an emitter and a collector, the
emitter of said fifth transistor being connected to the collector
of said fifth transistor through the resistor, the collector of
said fifth transistor is connected to the emitter of said first
transistor, and the base of said fifth transistor being connected
to a voltage in proportion to the time controller;
a sixth transistor having a base, an emitter and a collector, the
base of said sixth transistor being connected to the output voltage
of the n-lamp detector, the collector and emitter of said sixth
transistor being respectively connected to the base and emitter of
said third transistor and controlling the turn-off of said third
transistor according to the output voltage of the n-lamp
detector.
12. A ballast feedback control system, comprising:
a ballast which generates a feedback signal;
a detector which detects a number of lamps connected to the
ballast;
a reference voltage generator which produces a first output signal
corresponding to the number of lamps detected by the detector;
a soft start controller unit which produces a second output signal
corresponding to the number of lamps detected by the detector in an
initial preheat period, an instantaneous discharge period and a
continuous discharge period;
a feedback unit which receives said first output signal, said
second output signal and said feedback signal and generates a
control signal to be applied to said ballast.
13. The ballast feedback control system of claim 1, wherein said
control signal generated by said feedback unit has a frequency
which varies according to at least one of said first output signal,
said second output signal and said feedback signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a feedback control system for
ballast in a lighting system, and more specifically to a feedback
control system for lighting system ballast for lighting fixtures
such as a fluorescent lamps which detects the number of lamps
connected to the ballast and which uses an integrated circuit to
control the ballast.
2.Description of Related Art
A conventional lighting ballast will initially be described with
reference to the circuit diagram set forth in FIG. 1. As shown in
FIG. 1, a conventional ballast includes two switching transistors
M.sub.1, M.sub.2 connected together with diodes D.sub.1, D.sub.2
extending between their source and drain electrodes. Capacitors
C.sub.1, C.sub.2 and C.sub.4, C.sub.5 are connected across
transistors M.sub.1 and M.sub.2, and an inductor Lr and a lamp are
connected in series between a point of contact between capacitors
C.sub.1 and C.sub.2 and a point of contact between capacitors
C.sub.4 and C.sub.5. A capacitor C.sub.3 is connected to both ends
of the lamp.
A ballast having these elements is a switching type LC resonance
convertor. Driving signals Out.sub.1, Out.sub.2 are applied to
gates of the switching transistors M.sub.1 and M.sub.2 to thereby
control the path of current from direct link voltage E through the
lamp.
The on-off frequency of the switching transistors M.sub.1, M.sub.1
is called the switching frequency. The ballast can be operated in
an initial preheat mode, an instantaneous discharge mode and a
continuous discharge mode by controlling the switching
frequency.
The LC resonance frequency for a given ballast can be determined
through known equations assuming L is the inductance of the
inductor Lr and C is the equivalent capacitance of capacitors
C.sub.1 to C.sub.5.
In this ballast, if the switching frequency is controlled to be
higher than the LC resonance frequency, the power output from the
device varies in inverse proportion to the switching frequency.
Therefore, in the initial preheat mode, where relatively low power
is required, the switching frequency should be relatively high,
whereas in the continuous discharge mode, where full power is
required, the switching frequency should be lower.
There are two well known soft start ballast control systems:
feedforward control to detect input voltage and program control to
set a fixed driving frequency. One problem with soft start control
systems, however, is that they cannot control the ballast
accurately when there is a large change in external circumstances,
for example if there is a large change in input voltage. Further,
soft start control systems cannot control the ballast properly
during a load change, such as when the number of lamps changes and
may not work if the feedforward is not set properly.
SUMMARY OF THE INVENTION
The ballast control system of this invention provides frequency
control according to the number of lamps in the initial preheat
mode, instantaneous discharge mode and continuous discharge mode.
This ballast feedback control system provides many advantages--it
can control the ballast stably against irregular load
characteristics of the lamp, is energy efficient and prolongs the
effective life of the lamp.
An object of the present invention is to provide a continuous
feedback ballast control system which detects the number of lamps
in the system. More particularly, an object of this invention is to
provide a soft start signal and full output signal according to the
number of lamps in the soft start and full power mode through the
use of a feedback control system in order to overcome the
above-mentioned technical problems.
To achieve the above purposes, a switching type ballast control
system according to the present invention includes a detector to
detect the number of lamps, a reference voltage generator which
generates a reference voltage corresponding to the number of lamps
detected by detector and a soft start controller which produces
current corresponding to the number of lamps detected by the
detector. A feedback unit and a main control unit are provided
which add current generated by the feedback unit to a feedforward
current from the direct link voltage and determines a control
frequency of a driving signal of the ballast from this added
current.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the present invention will now be
described more specifically with reference to the attached
drawings, wherein:
FIG. 1 is a detailed circuit diagram of a conventional lighting
ballast circuit;
FIG. 2 is a block diagram of a lighting ballast control system
according to a preferred embodiment of the present invention;
FIG. 3 is a detailed circuit diagram of the control block of FIG.
2;
FIGS. 4 and 5 illustrate the current and power characteristics
controlled by the soft start controller of FIG. 2;
FIG. 6 is a detailed circuit diagram of the soft start controller
of FIG. 2;
FIG. 7 illustrates the current characteristic through the soft
start controller of FIG. 2;
FIG. 8 is a detailed circuit diagram of the n-lamp detector box of
FIG. 2, which detects the number of lamps in the ballast circuit;
and
FIGS. 9A to 9D are waveforms of output signals of the driving
circuit of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 2, a preferred ballast feedback control system of
this invention includes a ballast 1 to which a lamp is attached. A
n-lamp detector is provided which detects the number of lamps in
the circuit. A reference voltage generator 6 receives a signal n
indicative of the number of lamps in the circuit from the n-lamp
detector and generates a reference voltage. A soft start controller
4 also receives a signal n indicative of the number of lamps in the
circuit, and receives a signal from a time controller 41.
A direct link voltage E is applied to the ballast 1. The direct
link voltage and a feedback current ni.sub.fb from the ballast are
input to a multiplier 21, which produces an output current i.sub.mo
by multiplying those two input values. The output current i.sub.mo
may be expressed by the equation i.sub.mo =Km.times.ni.sub.fb
.times.E where Km is a multiplying constant. The output signal
i.sub.mo from the multiplier 21 is input to an adder 22.
A n-lamp detector detects the number of lamps attached to the
ballast 1 and outputs an output signal, the voltage of which varies
in accordance with the number of detected lamps. This output
voltage is input to a reference voltage generator 6 and a soft
start controller 4.
The reference voltage generator produces a reference voltage
nV.sub.ref corresponding to the output signal n from the n-lamp
detector. The reference voltage nV.sub.ref is used to determine the
input power of the ballast 1 and is input to the adder 24.
The soft start controller 4 produces a current signal nip from the
output signal n from the n-lamp detector and an output signal from
a time controller 41, which outputs a voltage in proportion to the
time. This output signal ni.sub.p is input to the adder 22. The
soft start controller 4 controls the magnitude of the output
current ni.sub.p which is required in the initial preheat period,
the instantaneous discharge period and the continuous discharge
period. A detailed explanation of this function follows.
An adder 22 adds the current ni.sub.p of the soft start controller
4 to the output signal i.sub.mo of the multiplier 21. This added
output signal i.sub.mol is input to a current to voltage convertor
23, which converts the input current i.sub.mol into voltage
V.sub.mo and outputs the voltage V.sub.mo to an adder 24.
Adder 24 produces an error voltage V.sub.err by subtracting the
output voltage V.sub.mo of the current to voltage convertor 23 from
the reference voltage nV.sub.ref of the reference voltage generator
6. The error voltage V.sub.err input to the voltage to current
convertor 25.
The voltage to current convertor 25 is formed of an error amplifier
which has transconductance Gm and which converts the input voltage
V.sub.err into the current i.sub.in. The current i.sub.in is input
to the control block 3.
The control block 3 produces a driving signal f.sub.1 for ballast 1
from the feedforward current i.sub.e e of the direct link voltage E
in inductor circuit 31, and the output current i.sub.in in from the
voltage to current convertor 25. The driving signal f.sub.1 is
input to the ballast 1.
The control block 3 has ballast switching elements switched
according to the driving signal f.sub.1 by determining the control
frequency of the driving signal f.sub.1. A detailed explanation on
the control block 3 will now be made with reference to FIG. 3,
which is a detailed circuit diagram of the control block 3.
As shown in FIG. 3, the control block 3 includes an integrator 311
which integrates the input current i.sub.in and a
voltage-controlled current source 312 which produces a current
i.sub.1 from the integrated voltage V.sub.in. An adder 313 produces
a total output current i.sub.t by adding an internal reference
current i.sub.ref and the feedforward current i.sub.e from the
inductor circuit 31, and subtracting the output current i.sub.1
from the voltage-controlled current source 312. An oscillator and
driving circuit 314 produce the driving signal f.sub.1 of the
ballast 1 from the total output current i.sub.t of the adder
313.
Operation of the control block 3 will now be explained. The output
current i.sub.in from the voltage to current convertor 25 is
integrated in integrator 311, which outputs the voltage V.sub.in to
a voltage-controlled current source 312. The voltage-controlled
current source 312 outputs the current i.sub.1 corresponding to the
voltage V.sub.in generated by integrator.
The output current i.sub.1 of voltage-controlled current source 312
is input to the adder 312 together with the output current i.sub.e
from the inductor circuit 31 and the internal reference current
i.sub.ref. The adder 313 adds the output current i.sub.e of
inductor circuit 31 to the reference current i.sub.ref and
subtracts the output current i.sub.1 of voltage controlled current
source 312, to produce the total current i.sub.t. This total
current i.sub.t is input to the oscillator and driving circuit
314.
The oscillator and driving circuit 314 outputs a driving signal
f.sub.1 by charging capacitor C.sub.t with the total current
i.sub.t, and determines the control frequency of the driving signal
f.sub.1.
The control frequency of the driving signal f.sub.1 determines the
input power of the ballast 1. This input power is proportionate to
the feedback current ni.sub.fb of ballast 1, which allows the
control system of the ballast to be controlled by feedback
control.
As discussed above, the reference voltage nV.sub.ref of the
reference voltage generator is used to determine the input
power.
The change of feedback current ni.sub.fb and direct link voltage E
which used to determine the voltage V.sub.mo are controlled so that
the output voltage V.sub.mo from the current to voltage convertor
23 is equal to the reference voltage nV.sub.re f.
Therefore in adder 22, if the current ni.sub.p of soft start
controller 4 increases, the output current i.sub.mo of multiplier
21 is reduced.
If the direct link voltage E is constant, the feedback current
ni.sub.fb is reduced. This reduction in feedback current ni.sub.fb
means that the control frequency of driving signal f.sub.1 in
control block is controlled to reduce the consumption of voltage of
the ballast system.
As explained above, the feedback control system of ballast is
applicable to the initial preheat mode. When feedback current
ni.sub.fb is reduced by increasing the output current ni.sub.p of
soft start controller 4, the feedback control system functions to
preheat the lamp which is in an undischarged condition.
After the requisite preheating is complete, feedback current
ni.sub.fb is controlled to produce the power required for discharge
by reducing the current ni.sub.p. During the continuous discharge
period, the current ni.sub.p is set to zero.
In this way, the ballast system is optimally controlled to provide
continuous feedback control in the initial preheat, instantaneous
discharge and continuous discharge periods.
FIGS. 4 and 5 respectively illustrate current and voltage
characteristics in the circuit when the current is controlled. As
shown in these Figs., the current ni.sub.p and the power nW.sub.p
are proportionately increased according to the number of lamps in
the circuit.
When the current ni.sub.p of the soft start controller 4 is
controlled to provide the current required for the initial preheat
period, the system power nW.sub.p of the ballast is controlled to
correspond to this current. For the instantaneous discharge period
after the initial preheat period, the current ni.sub.p is reduced
and the system power nW.sub.p is increased.
The feedback control system controls the current during the
instantaneous discharge period to ensure that there is sufficient
supplied power.
The continuous discharge period starts when the current ni.sub.p is
reduced to zero. The power level during the continuous discharge
period is the optimally controlled power of the ballast system.
FIG. 6 is a detailed circuit diagram of the soft start controller 4
and FIG. 7 illustrates current characteristics in the soft start
controller 4.
As shown in FIG. 6, the soft start controller 4 includes n-cells
411 to 41n, transistor Q.sub.7 and a current source 42 to supply
current for each cell.
The current source 42 and transistor Q.sub.7 supply current for
each cell through the use of a current mirror or current lens.
Since each cell is identical, the internal structure of only one
cell 411 will be explained in detail below.
In cell 411, the base of transistor Q.sub.6 is connected to the
base of transistor Q.sub.7. The emitter of transistor Q.sub.6 is
connected to the emitter of transistor Q.sub.7. The
emitter-collector current is proportional to the current of the
current source
The collector of the transistor Q.sub.6 is connected to the
collector of a transistor Q.sub.5, which has its base and collector
connected. The emitter of transistor Q.sub.5 is connected to the
current source 42.
The base of transistor Q.sub.5 is connected to the base of
transistor Q.sub.3. The base of transistor Q.sub.3 is connected to
the collector of transistor Q.sub.4. The output voltage of the
n-lamp detector 5 is applied to the base of transistor Q.sub.4. The
emitter of transistor Q.sub.3 is connected to the emitter of
transistor Q.sub.4. The transistor Q.sub.3 can be turned on when
the transistor Q.sub.4 is turned on by the output voltage of the
n-lamp detector.
As the transistor Q.sub.3 and transistor Q.sub.5 are mirrors of
each other, such that current in proportion to the current through
the collector of transistor Q.sub.6 flows through the collector of
transistor Q.sub.3.
Constant voltage V.sub.r2 applied to the base of the transistor
Q.sub.2 is applied to the collector of transistor Q.sub.3. The
collector of transistor Q.sub.3 is also connected to the emitter of
transistor Q.sub.2 which is connected to the adder 22. The
collector of transistor Q.sub.3 is supplied by the output voltage
V.sub.cs of time controller 41 and is connected to the emitter of
transistor Q.sub.1 which is connected to the collector of
transistor Q.sub.6 Resistor R.sub.1 is connected between the
emitter of transistor Q.sub.1 and the collector of transistor
Q.sub.3.
In this structure, the collector current of transistor Q.sub.2 is
input to adder 22. The voltage V.sub.cs in proportion to the time
of time controller 41 is input to the base of transistor Q.sub.1.
The output voltage of the n-lamp detector 5 to determine the
operation of appropriate cell 411 is input to the base of
transistor Q.sub.4.
The sum of collector current i.sub.p3 of transistor Q.sub.1 and
collector current i.sub.p2 of transistor Q.sub.2 is equivalent to
the collector current i.sub.p1 of transistor Q.sub.3. The collector
current of transistor Q.sub.3 is determined by the current source
42.
Collector current i.sub.p1 which is determined by the current
source 42 and the mirror or lens relation between transistors
Q.sub.6 and Q.sub.T, transistor Q.sub.3 and Q.sub.5 flows through
the collector of transistor Q.sub.3. At this time, the transistor
Q.sub.4 is turned on by the output voltage of the n-lamp detector
5, which turns transistors Q.sub.3 and Q.sub.5 on.
Transistor Q.sub.1 starts to turn on when the voltage V.sub.cs of
time controller 41 increases in proportion to time to be equal to
the voltage V.sub.r2 applied to the base of transistor Q.sub.2.
This moment corresponds to t.sub.1 4in FIG. 7.
Before t.sub.1, collector current i.sub.p1 of transistor Q.sub.3 is
nearly equivalent to the collector current i.sub.p2 of transistor
Q.sub.2 After t.sub.1, collector current i.sub.p3 of transistor
Q.sub.1 increases proportionally and collector current i.sub.p2
reduces proportionally. At this time the increasing slope of
current i.sub.p3 is inversely proportional to R.sub.1.
When the collector current i.sub.p3 of transistor Q.sub.1 is nearly
equal to the collector current i.sub.p1 of transistor Q.sub.3,
collector current i.sub.p2 of transistor Q.sub.2 becomes zero. This
moment correspond to t.sub.2 in FIG. 7.
As described above, a ballast can be controlled continuously by
controlling the collector current i.sub.p2 of transistor Q.sub.2
during the initial preheat, instantaneous discharge and continuous
discharge periods.
FIG. 8 is a detailed circuit of the n-lamp detector 5. As shown in
FIG. 8, a n-lamp detector 5 includes a comparator for each lamp and
an addition unit. When the voltage sensed by the comparators is
lower than a reference voltage V, the comparators output a voltage
V.sub.1amp.
The output voltage V.sub.1amp of each comparator is summed in the
addition unit to form an added voltage nV.sub.1map, which
corresponds to the number of lamps, and is input to the reference
voltage generator 6 and soft start controller 4.
Taking soft start controller 4 as an example, if there are 3 lamps
3V.sub.1amp is input to soft start controller 4 and V.sub.1amp is
input to 3 cells separately in soft start controller Consequently 3
cells in soft start controller can be active.
FIG. 9 illustrates signal waveforms of various parts of the circuit
shown in FIG. 3. FIG. 9A is a waveform of voltage charged in
capacitor C.sub.t which is connected to the oscillator and driving
circuit 314. FIG. 9B is a waveform of the output voltage of the
comparator in the oscillator and driving circuit 314. FIG. 9C and
9D are driving signals out.sub.1, out.sub.2 generated by the
oscillator and the driving circuit 314.
The driving signals out.sub.1, out.sub.2 are applied to the gate of
switching element in ballast 1. .DELTA.V as shown in FIG. 9A is the
magnitude of the sawtooth signal. The relation of total current
i.sub.t, the magnitude .DELTA.V of the sawtooth signal, control
frequency f.sub.1 of driving signal which is generated by control
block 3 and capacitance of capacitor C.sub.t is expressed by the
following equation,
which illustrates that the control frequency f.sub.1 is
proportional to the total current i.sub.t.
The dotted line as illustrated by FIG. 9A is a reference voltage of
the comparator in the oscillator and driving circuit 314. The
comparator output voltage waveform as shown in FIG. 9B is obtained
by comparing the dotted line with the sawtooth wave as shown in
FIG. 9A.
Comparator output voltage waveform as shown in FIG. 9B is divided
by the flipflop in the oscillator and driving circuit 314. These
divided signals, used to drive the ballast 1, are shown in FIG. 9C
and 9D. The waveforms as shown in FIG. 9C and 9D have frequency
f.sub.1 on the basis of one-side of waveform.
As described above, the present invention provides a ballast
feedback control system which can detect the number of lamps,
control the ballast continuously through the use of a n-lamp
detector and soft start controller which produces the compensated
current from the feedback current and direct link voltage.
Therefore, the feedback control system according to this invention
can control the ballast accurately against an external load change
such as a change of input voltage, or a change in the number of
lamps.
It is understood that various other modifications will be apparent
to and can be readily made by those skilled in the art without
departing from the scope and spirit of this invention. Accordingly,
it is not intended that the scope of the claims appended hereto be
limited to the description as set forth herein, but rather that the
claims be construed as encompassing all the features of patentable
novelty that reside in the present invention, including all
features that would be treated as equivalents thereof by those
skilled in the art which this invention pertains.
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